The present invention relates generally to a negative pressure control apparatus for an engine mounted in a vehicle.
In general, vehicle engines are powered by the combustion of a mixture of air supplied into a combustion chamber via the intake passage and fuel injected from the associated fuel injection valve. Located in the intake passage of the engine is a throttle valve for regulating the amount of air supplied to the combustion chamber. The engine power is controlled by regulating the amount of air supplied to the combustion chamber by adjusting the position, or angle, of the throttle valve.
Recently, an engine that switches the combustion mode in accordance with the running conditions of the engine to gain fuel efficiency and produce a high level of engine power has been proposed and put to practical use.
This type of engine gains adequate engine power by carrying out "homogeneous charge combustion," in which a homogeneous air-fuel mixture is burned. Homogeneous combustion is employed during high-speed or heavy-load operations. Homogeneous charge combustion is accomplished by homogeneously mixing the fuel injected into the combustion chamber with the air in the combustion chamber during the suction stroke of the engine and igniting the resultant air-fuel mixture with the ignition plug.
During low-speed or light-load operations, which do not require as much engine power, "stratified charge combustion" is executed, which increases the fuel concentration about the ignition plug to improve the ignitability and to make the average air-fuel ratio more lean than the stoichiometric air-fuel ratio, which reduces fuel consumption. Stratified charge combustion is carried out by causing fuel, which is injected into the combustion chamber during the compression stroke, to strike an indentation in the head of the associated piston such that the fuel gathers around the ignition plug. The mixture of the gathered fuel and the air in the combustion chamber is then ignited with the ignition plug. During stratified charge combustion, the throttle valve opening is regulated to be wider than that of homogeneous charge combustion to increase the average air-fuel ratio of the air-fuel mixture. This reduces the engine pumping loss.
Switching of the combustion mode of the engine between homogeneous charge combustion and stratified charge combustion in accordance with the running conditions of the engine improves fuel efficiency and provides appropriate engine power for the circumstances.
In an engine mounted in a vehicle, a brake booster, which reduces the force required to operate the brake pedal, is a negative-pressure, or vacuum, driven apparatus. That is, the brake booster operates using the vacuum pressure generated in the intake system. The brake booster stores vacuum generated in the intake system of the engine as the booster pressure, or working pressure, and operates based on the booster pressure. The booster pressure may be relied on to boost the brake force in an engine that executes stratified charge combustion. However, as mentioned previously, the throttle valve opening is wider during stratified charge combustion as compared with that in homogeneous charge combustion, and consequently, the negative pressure generated in the intake system of the engine approaches atmospheric pressure. Therefore, the booster pressure needed to operate the brake booster may not be available.
One conventional solution to this problem is to temporarily choke the throttle opening with the throttle valve to produce the booster pressure needed to operate the brake booster. The vacuum control apparatuses disclosed in, for example, Japanese Unexamined Patent Publication No. 10-151970 and Japanese Unexamined Patent Publication No. 10-167047 perform choking to create additional vacuum. The vacuum control apparatuses described in these publications choke the throttle when the booster pressure is inadequate to operate the brake booster. Choking the throttle reduces the pressure in the intake passage, thus producing the booster pressure needed to operate the brake booster.
When performing the choking control procedure, there is a delay between the time at which the throttle is choked and the time of the consequential change in the vacuum produced in the intake system. Thus, the time needed to produce the required booster pressure is extended by the response delay time. However, rapid vacuum production is needed to provide the required booster pressure as quickly as possible to improve the braking performance of the vehicle. According to the prior art, therefore, the opening of the throttle valve is relatively small during the normal stratified charge combustion to reduce the delay. Positioning the throttle valve this way during stratified charge combustion, that is, slightly closing the throttle during stratified charge combustion, causes the booster pressure to quickly reach the required value when the choking procedure is performed.
While slightly closing the throttle valve during normal stratified charge combustion hastens the production of the booster pressure needed to operate the brake booster, this scheme increases the engine pumping loss during normal stratified charge combustion, which increases fuel consumption.